[net] Impement a Mercury interface to getaddrinfo

Add a Mercury interface to the C function getaddrinfo.  getaddrinfo
superceeds gethostbyname and getservbyname.  It is reenterant and also makes
it easier to support IPv6 amoung other things.  Other predicates in netdb.m
should be written in terms of getaddrinfo.

extras/net/getaddrinfo.m:
    As above.

extras/net/netdb.m:
    Add predicates that use getaddrinfo to perform lookups.

extras/net/types.m:
    Add predicates to convert to and from the raw values for family and
    socktype.

extras/net/test_lookups.m:
extras/net/Makefile:
    Add a test program.

.gitignore:
    This .gitignore file was causing git to ignore the Mercury.options file
    in net/extras.  A leading slash in .gitignore patterns matches the start
    of a path, this allows us to ignore these patterns when they're only in
    the root directory.

extras/net/Mercury.options:
    Work around a Mercury bug.

    Add this missing file.

.gitignore

@@ -1,7 +1,7 @@
-README
-INSTALL
-TODO
-WORK_IN_PROGRESS
+/README
+/INSTALL
+/TODO
+/WORK_IN_PROGRESS
 configure
 config.cache
 config.status
@@ -10,15 +10,15 @@ configure.log
 configure.help
 confdefs.h
 so_locations
+/Mercury.options
 Mmake.common
 Mmake.params
 Mmake.stage.params
-stage1
-stage2
-stage3
-install_grade_dir.*
-Mercury.options
+/stage1
+/stage2
+/stage3
+/install_grade_dir.*
 autom4te.cache
 aclocal.m4
-.configured_library_grades
-main.o
+/.configured_library_grades
+/main.o