Rewrite the exec family.

* Removes duplication.
* Removes fixed PATH_MAX limit.
* Adds basic tests.

Bug: http://b/30032507
Change-Id: I4087a9de3e53884ac3297ebafbec4e6f8a9871b6
diff --git a/libc/Android.bp b/libc/Android.bp
index 5bfc930..589aeab 100644
--- a/libc/Android.bp
+++ b/libc/Android.bp
@@ -518,7 +518,6 @@
     name: "libc_openbsd_large_stack",
     defaults: ["libc_defaults"],
     srcs: [
-        "upstream-openbsd/lib/libc/gen/exec.c",
         "upstream-openbsd/lib/libc/stdio/vfprintf.c",
         "upstream-openbsd/lib/libc/stdio/vfwprintf.c",
         "upstream-openbsd/lib/libc/stdio/vfwscanf.c",
@@ -1238,6 +1237,7 @@
         "bionic/error.cpp",
         "bionic/eventfd_read.cpp",
         "bionic/eventfd_write.cpp",
+        "bionic/exec.cpp",
         "bionic/faccessat.cpp",
         "bionic/fchmod.cpp",
         "bionic/fchmodat.cpp",