MINOR: log: add lf_int() wrapper to print integers

MINOR: log: add lf_int() wrapper to print integers

Wrap ltoa(), lltoa(), ultoa() and utoa_pad() functions that are used by
sess_build_logline() to print numerical values by implementing a dedicated
helper named lf_int() that takes <dft_hld> as argument to know how to
write the integer by default (when no encoding is specified).

LF_INT_UTOA_PAD_4 is used to emulate utoa_pad(x, 4) since it's found only
once under sess_build_logline(), thus there is no need to pass an extra
parameter to lf_int() function.