string-util: add find_line[_after] functions

As a wrapper for `find_line_startswith`, `find_line_after` search for
the exact line and return the pointer for the next line, or NULL if
missing.

`find_line` with search for the exact line and return the pointer to the
beginning of the line.

Signed-off-by: Alberto Planas <aplanas@suse.com>

src/basic/string-util.h

@@ -279,6 +279,8 @@ char* strdupcspn(const char *a, const char *reject);
         })
 
 char* find_line_startswith(const char *haystack, const char *needle);
+char* find_line(const char *haystack, const char *needle);
+char* find_line_after(const char *haystack, const char *needle);
 
 bool version_is_valid(const char *s);