Modified GnuTLS priority according to standard crypto-policy guideline

[jvymazal]

Edited TLS priority string default to conform to system-wide default
and align librelp behavior with other crypto-utilizing packages

[jvymazal]

second attempt to solve #57

[jvymazal]

@rgerhards, as the code has been massively rewritten I canceled the original PR altogether and created this one instead where I re-implemented required changes onto the new code.

[codecov-io]

Codecov Report

Merging #171 into master will decrease coverage by 0.06%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master     #171      +/-   ##
==========================================
- Coverage    74.6%   74.54%   -0.07%     
==========================================
  Files          21       21              
  Lines        3950     3956       +6     
==========================================
+ Hits         2947     2949       +2     
- Misses       1003     1007       +4

Impacted Files	Coverage Δ
src/tcp.c	71.67% <83.33%> (-0.09%)	⬇️
src/sendbuf.c	66.66% <0%> (-7.58%)	⬇️
tests/send.c	78.96% <0%> (-1.72%)	⬇️
src/relp.c	77.65% <0%> (+0.54%)	⬆️
src/cserverclose.c	100% <0%> (+100%)	⬆️

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[rgerhards]

@jvymazal sorry for the veeeeery laaaaate reply - looks like I forgot to assign a milestone. But now it is finally part of the upcoming release :-)

[rgerhards]

@jvymazal not sure, but I guess the openssl driver may now work slightly different. Maybe you want to have a look to align that as well...