Reported by the new script 'scripts/copyright.pl'. The script has a
regex whitelist for the files that don't need copyright headers.
Removed three (mostly usesless) README files from docs/
Closes#5141
Note that for some reason there is this warning (that also exists with
autotools, added since curl-7_15_1-94-ga718cb05f):
docs/libcurl/curl_multi_socket_all.3:1: can't open `man3/curl_multi_socket.3': No such file or directory
Additionally, adjust the roffit --mandir option to support creating
links when doing out-of-tree builds.
Ref: https://github.com/curl/curl/pull/1288
Also make Perl mandatory to allow building the docs.
While CMakeLists.txt could probably read the list of manual pages from
Makefile.am, actually putting those in CMakeLists.txt is cleaner so that
is what is done here.
Fixes#1230
Ref: https://github.com/curl/curl/pull/1288
