posix_sockets: set read/write permissions for socket fd

[miri64]

Contribution description

In #11212 it was discovered, that the permissions for the file descriptors of POSIX sockets are not set so write and read can be used. This fixes that.

Testing procedure

Flash and run the application provided in #11212 on 2 samr21-xpros. Without this PR echo send returns "Bad file descriptor" as its write() call ends up here

RIOT/sys/vfs/vfs.c

Lines 325 to 328 in f6ee0ac

	if (((filp->flags & O_ACCMODE) != O_WRONLY) & ((filp->flags & O_ACCMODE) != O_RDWR)) {
	/* File not open for writing */
	return -EBADF;
	}

Issues/PRs references

Fixes #11212.

[miri64]

This bug fix should go into the release.

[haukepetersen]

ACK

[haukepetersen]

all green -> go

[miri64]

Thanks for the review :-)

[miri64]

@MrKevinWeiss as you are preparing for a 2019.07.1 release, shall I backport this one?

[MrKevinWeiss]

Sure we can add it to the list!