libmount: add a generic append_string() function

Signed-off-by: Karel Zak <kzak@redhat.com>
libmount: add functions to handle comments in fs tables

Co-Author: Karel Zak <kzak@redhat.com>
Signed-off-by: Ondrej Oprala <ooprala@redhat.com>
Signed-off-by: Karel Zak <kzak@redhat.com>
libmount: add functions to safely replace a fs table

Signed-off-by: Ondrej Oprala <ooprala@redhat.com>
tests: update libmount fstab full parse test

Signed-off-by: Karel Zak <kzak@redhat.com>