libmisc: Move xmalloc.c to alloc.c

We'll expand the contents in a following commit, so let's move the file
to a more generic name, have a dedicated header, and update includes.

Signed-off-by: Alejandro Colomar <alx@kernel.org>

Use the new header for xstrdup()

Signed-off-by: Alejandro Colomar <alx@kernel.org>

libmisc/Makefile.am

@@ -10,6 +10,7 @@ libmisc_la_SOURCES = \
 	addgrps.c \
 	age.c \
 	agetpass.c \
+	alloc.c \
 	audit_help.c \
 	basename.c \
 	bit.c \
@@ -78,7 +79,6 @@ libmisc_la_SOURCES = \
 	xgetgrnam.c \
 	xgetgrgid.c \
 	xgetspnam.c \
-	xmalloc.c \
 	yesno.c
 
 if WITH_BTRFS