Add local implementation of mntent.h

Bionic (at least) is missing some of the usual mntent functions.
This adds code defining those that we need when they're missing from the C
library.

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
conf.c: Cleanup __S_ISTYPE

__S_ISTYPE doesn't exist in all C libraries, so define it if it's missing.
Additionaly, replace one occurence where it wasn't actually needed.

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>