Cleanup: figured out why vstring_get() did not return
VSTREAM_EOF in APPEND mode.

Multiple 'bit rot' fixes for OpenSSL API changes, including
support to disable TLSv1.3, to avoid issuing multiple session
tickets, and to allow OpenSSL >= 1.1.0 run-time micro version
bumps without complaining about library version mismatches.

Feature: "postmap -F" reads a source file with (key, filename)
entries, and creates database records with (key, base64-encoded
filecontent). This feature will be used for SNI lookup
table support, where each key will be a domainname, and
each value will contain a sequence of (private key, certificate
hierarchy) for that domainname. The same 'value is filename'
behavior is implemented in cidr:, inline:, pcre:, randmap:,
regexp:, and static: maps if the application sets the flag
DICT_FLAG_RHS_IS_FILE. In the forseeable future, this will