Experimental pthread compatibility library

This patch adds pthread compatibility by using libmthread.

To use this with a program using pthreads, you have to replace
  #include <pthread>
with
  #define _MTHREADIFY_PTHREADS
  #include <minix/mthreads>

This also changes the initialization function to be a constructor, which
is implicitly called before the call to main. This allows for
conformance with pthreads, while not paying a high price by checking on
each mthread_* call whether the library has been initialized or not.

As mthread_init is now a constructor, it also has been set as static, and
relevent calls removed from programs using it.

Change-Id: I2aa375db557958d2bee9a70d285aabb990c88f00

share/mk/bsd.lib.mk

@@ -614,7 +614,7 @@ DPLIBC ?= ${DESTDIR}${LIBC_SO}
 .endif
 .else
 .if ${LIB} != "c" && ${LIB:Mgcc*} == "" \
-    && ${LIB} != "minlib" && ${LIB} != "sys" && ${LIB} != "minc" # Minix-specific libs
+    && ${LIB} != "minlib" && ${LIB} != "sys" && ${LIB} != "minc" && ${LIB} != "mthread" # Minix-specific libs
 .if !empty(LIBC_SO)
 DPLIBC ?= ${DESTDIR}${LIBC_SO}
 .endif

share/mk/bsd.prog.mk

@@ -221,6 +221,7 @@ LIB${_lib:tu}=	${DESTDIR}/usr/lib/lib${_lib:S/xx/++/:S/atf_c/atf-c/}.a
 	devman \
 	minc \
 	minlib \
+	mthread \
 	netdriver \
 	sffs \
 	sys \