Add `tls_ciphers` option to allow specifiying custom ciphers list

[plan44]

This is needed in particular when running pagekite.py as a frontend server on debian >=11, where the cipherlist must be set to DEFAULT@SECLEVEL=1 to allow current (at the time of writing this) libpagekite embedded backends to connect, as these use TLSv1 even on platforms which do support TLSv2 and later.

Also, this PR adds a help text for tls_legacy (which existed, but was not listed in help so far)