MEDIUM: ssl: replace "crt" lines by "ssl-f-use" lines

MEDIUM: ssl: replace "crt" lines by "ssl-f-use" lines

The new "crt" lines in frontend and listen sections are confusing:

- a filename is mandatory but we could need a syntax without the
  filename in the future, if the filename is generated for example
- there is no clue about the fact that its only used on the frontend
  side when reading the line

A new "ssl-f-use" line replaces the "crt" line, but a "crt" keyword
can be used on this line. "f" indicates that this is the frontend
configuration, a "ssl-b-use" keyword could be used in the future.

The "crt" lines only appeared in 3.2-dev so this won't change anything
for people using configurations from previous major versions.