Add FAQ, and FAQ entry for flags stripped at library link time.

Add FAQ, and FAQ entry for flags stripped at library link time.

* doc/libtool.texi (FAQ, Stripped link flags): New nodes.
(Wrapper executables): Replace bug address with macro.
(Compile mode, Link mode): Document `-Wc,' and `-Xcompiler '
semantics better.
* libltdl/config/ltmain.m4sh (func_mode_link): Rewrite comment
for the flags that are passed through.
Prompted by reports from Simon Richter <Simon.Richter@hogyros.de>,
Török Edwin <edwintorok@gmail.com> and
Åke Sandgren <ake.sandgren@hpc2n.umu.se>.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>