* src/utils.h: Add struct file_stat_s declaration,
change prototypes of file_exists_p(),
add prototypes for fopen_stat() and open_stat().
* src/utils.c: Extend file_exists_p(),
new function fopen_stat() and open_stat(),
add new param for file_exists_p().
* src/init.h: Add param file_stats_t to run_wgetrc().
* src/ftp.c: Amend calls to extended functions.
* src/hsts.c: Likewise.
* src/http.c: Likewise.
* src/init.c: Likewise.
* src/main.c: Likewise.
* src/metalink.c: Likewise.
* src/retr.c: Likewise.
* src/url.c: Likewise.
Added fopen_stat() and open_stat() that checks to makes sure the file didn't
change underneath us.
Return error from file_exists_p().
Added a way to return error from this file without major surgery to the
callers.
Fixes: #20369
