nasmlib: add function to splice pathnames

nasmlib: add function to splice pathnames

Add a function to splice a pathname consisting of a directory and a
filename.  It is worth noting that this function is limited to that
particular use case: in particular, it does NOT currently support
concatenating a filename which itself contains directory components to
a non-null directory.

Combining directory names is extremely system-dependent and probably
needs more than just parameterized code in many cases, for example,
on VMS combining "foo:[bar]" with "[baz]quux" should produce
"foo:[bar.baz]quux" whereas combining "foo:[bar]" and baz:quux" is an
outright error.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>