Let SQLite use pread/pwrite

[navytux]

With current settings SQLite was using lseek/read syscalls to read data, e.g.:

20:43:17.640660 fcntl(3, F_SETLK, {l_type=F_UNLCK, l_whence=SEEK_SET, l_start=0, l_len=0}) = 0
20:43:17.640683 fcntl(3, F_SETLK, {l_type=F_RDLCK, l_whence=SEEK_SET, l_start=1073741824, l_len=1}) = 0
20:43:17.640705 fcntl(3, F_SETLK, {l_type=F_RDLCK, l_whence=SEEK_SET, l_start=1073741826, l_len=510}) = 0
20:43:17.640725 fcntl(3, F_SETLK, {l_type=F_UNLCK, l_whence=SEEK_SET, l_start=1073741824, l_len=1}) = 0
20:43:17.640744 stat(".../neo.sqlite-journal", 0x7ffef2c91080) = -1 ENOENT (No such file or directory)
20:43:17.640764 lseek(3, 24, SEEK_SET)  = 24
20:43:17.640779 read(3, "\0\0\0\33\0\0\10\235\0\0\10]\0\0\0\27", 16) = 16
20:43:17.640795 stat(".../neo.sqlite-wal", 0x7ffef2c91080) = -1 ENOENT (No such file or directory)

but if we allow it to use pread it will be only 1 system call instead of 2 and
reading this way can also be done in parallel because there is no global to
file seeking:

20:48:42.668466 fcntl(3, F_SETLK, {l_type=F_UNLCK, l_whence=SEEK_SET, l_start=0, l_len=0}) = 0
20:48:42.668501 fcntl(3, F_SETLK, {l_type=F_RDLCK, l_whence=SEEK_SET, l_start=1073741824, l_len=1}) = 0
20:48:42.668522 fcntl(3, F_SETLK, {l_type=F_RDLCK, l_whence=SEEK_SET, l_start=1073741826, l_len=510}) = 0
20:48:42.668542 fcntl(3, F_SETLK, {l_type=F_UNLCK, l_whence=SEEK_SET, l_start=1073741824, l_len=1}) = 0
20:48:42.668561 stat(".../neo.sqlite-journal", 0x7ffdbc1f22c0) = -1 ENOENT (No such file or directory)
20:48:42.668580 pread64(3, "\0\0\0\33\0\0\10\235\0\0\10]\0\0\0\27", 16, 24) = 16
20:48:42.668597 stat(".../neo.sqlite-wal", 0x7ffdbc1f22c0) = -1 ENOENT (No such file or directory)

(if needed this enablement can be done per OS)

[coveralls]

Coverage remained the same at 68.639% when pulling f675967 on navytux:y/preadwrite into 3c6eef4 on mattn:master.

[mattn]

Thank you

[navytux]

Thanks for merging.