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Properly escape commands in pmb.chroot.user() (#1316)

## Introduction
In #1302 we noticed that `pmb.chroot.user()` does not escape commands
properly: When passing one string with spaces, it would pass them as
two strings to the chroot. The use case is passing a description with
a space inside to `newapkbuild` with `pmboostrap newapkbuild`.

This is not a security issue, as we don't pass strings from untrusted
input to this function.

## Functions for running commands in pmbootstrap
To put the rest of the description in context: We have four high level
functions that run commands:
* `pmb.helpers.run.user()`
* `pmb.helpers.run.root()`
* `pmb.chroot.root()`
* `pmb.chroot.user()`

In addition, one low level function that the others invoke:
* `pmb.helpers.run.core()`

## Flawed test case
The issue described above did not get detected for so long, because we
have a test case in place since day one, which verifies that all of the
functions above escape everything properly:
* `test/test_shell_escape.py`

So the test case ran a given command through all these functions, and
compared the result each time. However, `pmb.chroot.root()`
modified the command variable (passed by reference) and did the
escaping already, which means `pmb.chroot.user()` running directly
afterwards only returns the right output when *not* doing any escaping.

Without questioning the accuracy of the test case, I've escaped
commands and environment variables with `shlex.quote()` *before*
passing them to `pmb.chroot.user()`. In retrospective this does not
make sense at all and is reverted with this commit.

## Environment variables
By coincidence, we have only passed custom environment variables to
`pmb.chroot.user()`, never to the other high level functions. This only
worked, because we did not do any escaping and the passed line gets
executed as shell command:
```
$ MYENV=test echo test2
test 2
```
If it was properly escaped as one shell com... (continued)

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Jobs
ID Job ID Ran Files Coverage
1 4373.1 81
76.28
 Travis Job 4373.1
Source Files on build 4373