Remove unnecessary "extern" from function declarations

[lrstewart]

Resolved issues:

resolves #1618

Description of changes:

"extern" is meaningless when applied to functions. Function declarations are "extern" by default.

I applied the basic replacement s/extern (.*) (.*)\(/$1 $2(/ to the codebase to remove the externs from function declarations. The \( in the regex prevents it from applying to variables, where "extern" actually has meaning.

Call-outs:

All these changes were done automatically (except for the new check in grep_simple_mistakes.sh).

Testing:

grep_simple_mistakes.sh passes.

If I add back one extern to s2n_tls13.h, it fails with:

Found unnecessary 'extern' in function declaration
/home/lrstewart/Code/s2n-tls/tls/s2n_tls13.h:extern bool s2n_use_default_tls13_config();
FAILED Grep For Simple Mistakes check

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[dougch]

zsh is happy