Put special pipe-handling code into libwsutil.

Put special pipe-handling code into libwsutil.

Ask, in a comment, why we're doing PeekNamedPipe() when we're trying
to read everyting in the pipe, up to the EOF, into a string.

On UN*X, do the same "read up to an EOF and then NUL-terminate the
result" stuff that we did on Windows; nothing guarantees that, on all
UN*Xes, in all circumstances, until the end of time, world without end,
amen, we can do one read and get the entire string.

Change-Id: I578802b23fec1051139eaefd9a09fe2a6de06a11
Reviewed-on: https://code.wireshark.org/review/24959
Petri-Dish: Guy Harris <guy@alum.mit.edu>
Tested-by: Petri Dish Buildbot
Reviewed-by: Guy Harris <guy@alum.mit.edu>