MINOR: h3: do not consider missing buf room as error on trailers

MINOR: h3: do not consider missing buf room as error on trailers

Improve h3_resp_trailers_send() return value to be similar with
h3_resp_data_send(). In particular, if QCS Tx buffer has not enough
space for trailer encoding, 0 is returned instead of an error value,
with QC_SF_BLK_MROOM set.

This unify HTTP/3 headers/data/trailers encoding functions. Negative
error codes are limited to fatal error which should cause a connection
closure. Not enough output buffer space is only a transient condition
which is reflect by the QC_SF_BLK_MROOM flag.