Detect the presence of pread()/pwrite() system calls on unix systems and

use them if available.

FossilOrigin-Name: 82cbf5ad2ed988a006106e188e98096a52e6779d

diff --cc manifest
index d5a273c8dfc6b4f00056f7c5f547ebec90b377b6,be929a8bc70d0f89cfa9bae441d50dfb62194a1f..5f2a2227240c78427498fa89843635defeb7c765
--- 1/manifest
--- 2/manifest
+++ b/manifest
@@@ -1,8 -1,8 +1,8 @@@
- C Add\sthe\stest_bestindex.c\sfile\sto\sMakefile.in\sand\sMakefile.msc\sso\sthat\n"make\stest"\swill\swork\sagain.
- D 2016-03-04T02:59:35.823
 -C Further\sfixes\sto\sthe\spread()/pwrite()\senabling\slogic.
 -D 2016-03-04T03:14:39.390
 -F Makefile.in e335453db0b16da00c884ad51bb56d1c091a74de
++C Detect\sthe\spresence\sof\spread()/pwrite()\ssystem\scalls\son\sunix\ssystems\sand\nuse\sthem\sif\savailable.
++D 2016-03-04T03:20:05.475
 +F Makefile.in 4c3d3d1b98454bf60e8977223b26ef30777a2bf2
  F Makefile.linux-gcc 7bc79876b875010e8c8f9502eb935ca92aa3c434
 -F Makefile.msc dbd4621ecc585c2ef0c2aa0874698c54675754f1
 +F Makefile.msc 0dc13352b84b257d81736076d8101a584940c409
  F README.md 8ecc12493ff9f820cdea6520a9016001cb2e59b7
  F VERSION c6b1f51809551d60ad001e6d87cf3ab2c7f54b6f
  F aclocal.m4 a5c22d164aff7ed549d53a90fa56d56955281f50
@@@ -1453,7 -1453,7 +1453,8 @@@ F vsixtest/vsixtest.tcl 6a9a6ab600c25a9
  F vsixtest/vsixtest.vcxproj.data 2ed517e100c66dc455b492e1a33350c1b20fbcdc
  F vsixtest/vsixtest.vcxproj.filters 37e51ffedcdb064aad6ff33b6148725226cd608e
  F vsixtest/vsixtest_TemporaryKey.pfx e5b1b036facdb453873e7084e1cae9102ccc67a0
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diff --cc manifest.uuid
index d8612046f6fea44d6efab5edaffd80c951075c47,0966145f3c7d25ad8bc9450fbf3b61e9c3af372c..a3b88f39bdc859c5e72b018f676b9d8802edf950
--- 1/manifest.uuid
--- 2/manifest.uuid
+++ b/manifest.uuid
@@@ -1,1 -1,1 +1,1 @@@
- 6ec6b6a97e62be1abe1e349b5bc04c6f5ae88968
 -38fb54e7854ac0500a88585ff9dd7487b4b2e586
++82cbf5ad2ed988a006106e188e98096a52e6779d