* Makefile.am: Added new source files hsts.c and hsts.h.
* http.c (parse_strict_transport_security): new function for STS header
parsing.
(gethttp): update the HSTS store.
* http.h: new include "hsts.h".
* init.c: new options --hsts and --hsts-file.
* main.c (get_hsts_database, load_hsts, save_hsts): new functions.
New options --no-hsts and --hsts-file added to help.
(main): load and save HSTS store.
* options.h: new variables for supporting --hsts and --hsts-file.
* retr.c (retrieve_url): rewrite the URI according to the HSTS policy before
entering http_loop.
* test.c, test.h: new unit tests for HSTS.
* utils.c, utils.h (countchars): new function.
* wget.h: new preprocessor check.
* hsts.c, hsts.h: new files with the HSTS engine implementation.
Added support for HTTP Strict Transport Security (HSTS), as defined by RFC
6797.
