If the global TLS initialisation could not been set up,
tls_is_available() returns 0, otherwise 1.
It is not very useful calling any other tls_*-efun if this one
returns 0, since there is no TLS-encryption available.
Most likely the global initialisation fails due to missing or
unreadable key resp. certificate-file.
Introduced in LDMud 3.3.474 and following, backported to 3.2.11.