input: pass larger buffer to auto format detection

input: pass larger buffer to auto format detection

The previous implementation already was inconsistent (used to allocate
255 bytes and claimed that 128 bytes were sufficient). While existing
formats already required more than a few bytes of input (regular VCD
files' header sections exceed 255 bytes length).

Increase the buffer size that gets passed to input modules' match
method. Use 4MiB for consistency with other locations. Do not enforce a
minimum size, as there are valid input files which are shorter than 128
bytes. Auto-detection failed on those.

This addresses part of bug #1200.