Deps log format v3
This fixes two bugs.
1.) It makes the record size field 4 byte instead of 2, and allows a max
record size of 512kB. This fixes #605, ninja "only" supporting 8k
dependencies per file -- 512kB have room for 128k dependencies. (If that's
still not enough, the current design can support at least 4x that if the
constant is tweaked.)
2.) It makes all records 4-byte aligned, which is probably needed to make the
`reinterpret_cast<int*>(buf)` work on SPARC (cf issue #481), and it's
generally nicer too. (Since there's already one reinterpret_cast, convert
a memcpy() to reinterpret_cast in another branch too).