dnsdist: Prevent allocations in two corner cases

[rgacogne]

Short description

• Avoid a reallocation if the initial allocation of the proxy header is too small to host the remaining content
• When syslog logging is enabled, we do not need to allocate two strings with the same content to log to syslog and stdout

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)