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/basic/fileio.c

@@ -784,7 +784,9 @@ static int merge_env_file_push(
         }
 
         expanded_value = replace_env(value, *env,
-                                     REPLACE_ENV_USE_ENVIRONMENT|REPLACE_ENV_ALLOW_BRACELESS);
+                                     REPLACE_ENV_USE_ENVIRONMENT|
+                                     REPLACE_ENV_ALLOW_BRACELESS|
+                                     REPLACE_ENV_ALLOW_EXTENDED);
         if (!expanded_value)
                 return -ENOMEM;
 
@@ -799,7 +801,7 @@ int merge_env_file(
                 const char *fname) {
 
         /* NOTE: this function supports braceful and braceless variable expansions,
-         * unlike other exported parsing functions.
+         * plus "extended" substitutions, unlike other exported parsing functions.
          */
 
         return parse_env_file_internal(f, fname, NEWLINE, merge_env_file_push, env, NULL);