add DESTDIR, MANDIR to install target

DESTDIR to allow installing in chroot
MANDIR to allow specifying different dir for man pages
Merge pull request #34 from swills/master

add DESTDIR, MANDIR to install target
Update documentation after merge of #34
Use NULL instead of '\0' to avoid type conversion
Merge pull request #32 from fix-macosx/macosx-yosemite

Support Mac OS X 10.10 by using 10.9 headers
Update documentation after merge of pull req #32
Check return values of pthread_mutex_init and friends

Issue:		#38
Reported by:	Markus Elfring
Handle strdup() failure in early stages of main()

Issue:		#38
Reported by:	Markus Elfring
Merge pull request #35 from fix-macosx/specify-custom-gid

Add support for specifying an explicit group when dropping privileges.
Further improving OOM handling in early stages of main()
Slightly improve user experience for new option -m
Update documentation after merging pull req #35
Update TODO and refer to github issues
Disable SSLv2 support by default
Don't depend on the space when parsing HTTP headers
Update khash.h to latest klib master
Rewrite Mac OS X support to use proper XNU headers

Move from one set of headers per major OS X release to one set of
headers per XNU release.  Fetch the header files from Apple's official
Open Source site instead of GitHub in the fetchdeps developer target.
As a side effect, 10.6.x is now supported as well (untested), and proper
headers are used for 10.10.

Issue:		#39
Filter HSTS response headers to allow cert override

Also remove HTTP Strict Transport Security (HSTS, RFC 6797) headers from
HTTP responses.  With HSTS active, the user is not allowed to accept
untrusted certificates.
Improve manual page re protocols and scalability

Issue:		#42
Make SSLv2 support opt-in instead of opt-out

Migrate knobs from DISABLE_SSLV2_CLIENT and DISABLE_SSLV2_SERVER to
WANT_SSLV2_CLIENT and WANT_SSLV2_SERVER and remove the
DISABLE_SSLV2_SESSION_CACHE knob entirely, automatically including the
respective code if SSLv2 support is compiled into SSLsplit.