Add support for content-disposition

* module/web/http.scm ("Content-Disposition"): Add a parser and
  serializer.  Defined in RFC2616 section 19.5.1.

* test-suite/tests/web-http.test ("entity headers"): New test case.
Write out HTTP Basic auth headers correctly.

Fixes <http://bugs.gnu.org/14370>.
Reported by Atom X Zane <atomx@deadlyhead.com>.

* module/web/http.scm (write-credentials): Handle the Basic auth scheme
  correctly.

* test-suite/tests/web-http.test (pass-if-round-trip): Use
  'pass-if-equal' for better error reporting.
  ("request headers"): Add tests.

* THANKS: Add "Atom X Zane" to bug fix section.
REPL Server: Don't establish a SIGINT handler.

* module/system/repl/server.scm (call-with-sigint): Remove.
  (accept-new-client): Don't wrap 'call-with-sigint' around call to
  'accept'.
Fix nested block comment example in manual.

Reported by Chris K. Jester-Young <cky944@gmail.com>.

* doc/ref/api-evaluation.texi (Block Comments): Fix example.
Buffered custom binary input ports correctly handle partial read requests.

* libguile/r6rs-ports.c (cbip_fill_input): Always initialize 'read_pos'
  when BUFFERED.
* test-suite/tests/r6rs-ports.test ("7.2.7 Input Ports")["custom binary
  input port buffered partial reads"]: New test.