glib-unix: Add g_unix_get_passwd_entry() function

This is a convenience wrapper around getpwnam_r() which handles all the
memory allocation faff.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Helps: #1687

docs/reference/glib/glib-sections.txt

@@ -2455,6 +2455,9 @@ g_unix_fd_add
 g_unix_fd_add_full
 g_unix_fd_source_new
 
+<SUBSECTION>
+g_unix_get_passwd_entry
+
 <SUBSECTION Private>
 g_unix_error_quark
 </SECTION>