BUG/MEDIUM: http: Prevent replace-header from overwriting a buffer

BUG/MEDIUM: http: Prevent replace-header from overwriting a buffer

This is the same fix as which concerning the redirect rules (0d94576c).

The buffer used to expand the <replace-fmt> argument must be protected to
prevent it being overwritten during build_logline() execution (the function used
to expand the format string).

This patch should be backported in 1.7, 1.6 and 1.5. It relies on commit b686afd
("MINOR: chunks: implement a simple dynamic allocator for trash buffers") for
the trash allocator, which has to be backported as well.