Updated and corrected build instructions.

[rincebrain]

While adding python{,3}-packaging to the build requirements, I
suspected that the build instructions hadn't actually been tested
recently. So I checked, found problems, and corrected them.

Briefly:

• --skip-broken is needed in a number of commands to ignore the absence of EPEL/powertools on Fedora
• Broke out the install commands that specifically required EPEL/powertools; the alternative was breaking out the command to install EPEL, I have no strong opinion on which one to prefer.
• I had to break out the Debian/Ubuntu install step because Debian calls it linux-perf-KERNELVER with a linux-perf metapackage, while Ubuntu calls it linux-tools-common, neither has a compat package for the other's name, and apt appears to have no equivalent for --skip-broken to just specify both.
  (The alternative of putting a shared Debian/Ubuntu command and then additional Debian/Ubuntu specific steps below it seemed to me to be likely to cause confusion; I can do that instead if people disagree.)
• EPEL is needed for python-packaging on EL7 (powertools, which ships disabled with the OS, is needed on CentOS8; I do not have an actual RHEL8 system to examine the state there)
• We still need --skip-broken for the command installing pax because it's (AFAICT) entirely absent on EL8

I've tested these instructions with:

• Ubuntu 21.04 x64
• Fedora 34 x64
• Debian 11 mips64el
• CentOS 8 x64
• CentOS 7 x64

And it successfully built {deb,rpm}-{kmod,dkms,utils} as appropriate (except on mips64el, but that seems to just be broken on the platform, not a problem of a missing dependency)

(If openzfs/zfs#12102 lands, I can instead drop the -packaging additions and rewrite the checks from openzfs/zfs#12073 that are the whole reason for adding the requirement...)

[gmelikov]

LGTM, thanks!