basic: support default and alternate values for env expansion

Sometimes it's useful to provide a default value during an environment
expansion, if the environment variable isn't already set.

For instance $XDG_DATA_DIRS is suppose to default to:

/usr/local/share/:/usr/share/

if it's not yet set. That means callers wishing to augment
XDG_DATA_DIRS need to manually add those two values.

This commit changes replace_env to support the following shell
compatible default value syntax:

XDG_DATA_DIRS=/foo:${XDG_DATA_DIRS:-/usr/local/share/:/usr/share}

Likewise, it's useful to provide an alternate value during an
environment expansion, if the environment variable isn't already set.

For instance, $LD_LIBRARY_PATH will inadvertently search the current
working directory if it starts or ends with a colon, so the following
is usually wrong:

LD_LIBRARY_PATH=/foo/lib:${LD_LIBRARY_PATH}

To address that, this changes replace_env to support the following
shell compatible alternate value syntax:

LD_LIBRARY_PATH=/foo/lib${LD_LIBRARY_PATH:+:${LD_LIBRARY_PATH}}

[zj: gate the new syntax under REPLACE_ENV_ALLOW_EXTENDED switch, so
existing callers are not modified.]

src/test/test-env-util.c

@@ -185,6 +185,12 @@ static void test_replace_env_argv(void) {
                 "${FOO",
                 "FOO$$${FOO}",
                 "$$FOO${FOO}",
+                "${FOO:-${BAR}}",
+                "${QUUX:-${FOO}}",
+                "${FOO:+${BAR}}",
+                "${QUUX:+${BAR}}",
+                "${FOO:+|${BAR}|}}",
+                "${FOO:+|${BAR}{|}",
                 NULL
         };
         _cleanup_strv_free_ char **r = NULL;
@@ -202,7 +208,13 @@ static void test_replace_env_argv(void) {
         assert_se(streq(r[8], "${FOO"));
         assert_se(streq(r[9], "FOO$BAR BAR"));
         assert_se(streq(r[10], "$FOOBAR BAR"));
-        assert_se(strv_length(r) == 11);
+        assert_se(streq(r[11], "${FOO:-waldo}"));
+        assert_se(streq(r[12], "${QUUX:-BAR BAR}"));
+        assert_se(streq(r[13], "${FOO:+waldo}"));
+        assert_se(streq(r[14], "${QUUX:+waldo}"));
+        assert_se(streq(r[15], "${FOO:+|waldo|}}"));
+        assert_se(streq(r[16], "${FOO:+|waldo{|}"));
+        assert_se(strv_length(r) == 17);
 }
 
 static void test_env_clean(void) {

src/test/test-fileio.c

@@ -229,6 +229,10 @@ static void test_merge_env_file(void) {
                                 "twentytwo=2${one}\n"
                                 "xxx_minus_three=$xxx - 3\n"
                                 "xxx=0x$one$one$one\n"
+                                "yyy=${one:-fallback}\n"
+                                "zzz=${one:+replacement}\n"
+                                "zzzz=${foobar:-${nothing}}\n"
+                                "zzzzz=${nothing:+${nothing}}\n"
                                 , false);
         assert(r >= 0);
 
@@ -245,7 +249,11 @@ static void test_merge_env_file(void) {
         assert_se(streq(a[3], "twentytwo=22"));
         assert_se(streq(a[4], "xxx=0x222"));
         assert_se(streq(a[5], "xxx_minus_three= - 3"));
-        assert_se(a[6] == NULL);
+        assert_se(streq(a[6], "yyy=2"));
+        assert_se(streq(a[7], "zzz=replacement"));
+        assert_se(streq(a[8], "zzzz="));
+        assert_se(streq(a[9], "zzzzz="));
+        assert_se(a[10] == NULL);
 
         r = merge_env_file(&a, NULL, t);
         assert_se(r >= 0);
@@ -260,7 +268,11 @@ static void test_merge_env_file(void) {
         assert_se(streq(a[3], "twentytwo=22"));
         assert_se(streq(a[4], "xxx=0x222"));
         assert_se(streq(a[5], "xxx_minus_three=0x222 - 3"));
-        assert_se(a[6] == NULL);
+        assert_se(streq(a[6], "yyy=2"));
+        assert_se(streq(a[7], "zzz=replacement"));
+        assert_se(streq(a[8], "zzzz="));
+        assert_se(streq(a[9], "zzzzz="));
+        assert_se(a[10] == NULL);
 }
 
 static void test_merge_env_file_invalid(void) {