Enable linknamespace testing for libdl and libcrypt.

When I set up linknamespace testing, the lists of libraries that might
contain functions from various standards were based on the -l options
POSIX says may be required to find certain functions with the c99
utility.

glibc has some POSIX functions in the libdl and libcrypt libraries,
not mentioned in the definition of the c99 utility (so an
implementation of that utility using glibc would need to use -ldl
-lcrypt automatically).  This patch adds those libraries to the ones
considered in linknamespace testing for relevant standards.  (The
crypt functions are XSI only, present in XPG3 and above; the libdl
ones were added in UNIX98, then moved from XSI to BASE in the 2008
edition of POSIX.)

	* conform/Makefile (linknamespace-libs): Rename to
	linknamespace-libs-thr.
	(linknamespace-libs-posix): New variable.
	(linknamespace-libs-xsi): Likewise.
	(linknamespace-libs-XPG3): Include libcrypt.a.
	(linknamespace-libs-XPG4): Use $(linknamespace-libs-XPG3).
	(linknamespace-libs-POSIX): Use $(linknamespace-libs-thr).
	(linknamespace-libs-UNIX98): Use $(linknamespace-libs-xsi).
	(linknamespace-libs-XOPEN2K): Likewise.
	(linknamespace-libs-XOPEN2K8): Likewise.
	(linknamespace-libs-POSIX2008): Use $(linknamespace-libs-posix).