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/env-util.h

@@ -32,6 +32,7 @@ bool env_assignment_is_valid(const char *e);
 enum {
         REPLACE_ENV_USE_ENVIRONMENT = 1u,
         REPLACE_ENV_ALLOW_BRACELESS = 2u,
+        REPLACE_ENV_ALLOW_EXTENDED  = 4u,
 };
 
 char *replace_env_n(const char *format, size_t n, char **env, unsigned flags);