basic: add new merge_env_file function

merge_env_file is a new function, that's like load_env_file, but takes a
pre-existing environment as an input argument. New environment entries are
merged. Variable expansion is performed.

Falling back to the process environment is supported (when a flag is set).
Alternatively this could be implemented as passing an additional fallback
environment array, but later on we're adding another flag to allow braceless
expansion, and the two flags can be combined in one arg, so there's less
stuff to pass around.

src/basic/env-util.h

@@ -29,7 +29,11 @@ bool env_name_is_valid(const char *e);
 bool env_value_is_valid(const char *e);
 bool env_assignment_is_valid(const char *e);
 
-char *replace_env(const char *format, char **env);
+enum {
+        REPLACE_ENV_USE_ENVIRONMENT = 1u,
+};
+
+char *replace_env(const char *format, char **env, unsigned flags);
 char **replace_env_argv(char **argv, char **env);
 
 bool strv_env_is_valid(char **e);
@@ -47,7 +51,7 @@ char **strv_env_unset(char **l, const char *p); /* In place ... */
 char **strv_env_unset_many(char **l, ...) _sentinel_;
 int strv_env_replace(char ***l, char *p); /* In place ... */
 
-char *strv_env_get_n(char **l, const char *name, size_t k) _pure_;
+char *strv_env_get_n(char **l, const char *name, size_t k, unsigned flags) _pure_;
 char *strv_env_get(char **x, const char *n) _pure_;
 
 int getenv_bool(const char *p);