Fix a segfault in parsing OpenSSH private key files.

Fix a segfault in parsing OpenSSH private key files.

The initial test for a line ending with "PRIVATE KEY-----" failed to
take into account the possibility that the line might be shorter than
that. Fixed by introducing a new library function strendswith(), and
strstartswith() for good measure, and using that.

Thanks to Hanno Böck for spotting this, with the aid of AFL.

(cherry picked from commit fa7b23ce9025daba08e86bb934fc430099792b9a)

Conflicts:
	misc.c
	misc.h

(cherry-picker's note: the conflicts were only due to other functions
introduced on trunk just next to the ones introduced by this commit)