Simplify concatenation of formatted strings

[killenheladagen]

Instead of keeping track of offset into and length of buffer, keep track of pointers to next input and to end of buffer. This means one less subtraction in every call to g_snprintf() and occationally statements that don't have to span several lines.

The --tlsverify flag was also moved from tls.c to the main file.

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• I have performed a self-review of my own code
• I have verified that the code builds perfectly fine on my local system
• I have added tests that prove my fix is effective or that my feature works
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• I have verified that my code follows the style already available in the repository
• I have made corresponding changes to the documentation